Composition and method of forming aluminum alloy foil

ABSTRACT

The present invention provides an aluminum alloy foil for fins used in heat exchangers. The aluminum alloy composition consists essentially of about 0.25% to about 0.6% by weight of Si; about 0.15% to about 0.50% by weight of Fe; about 0.20% to about 0.70% by weight of Mn; less than about 0.05% Cu; and less than about 0.05% Mg, with the balance aluminum including unavoidable impurities. The alloy composition may also contain less than 0.10% Zn or 0.50-2.00% Zn. The invention also provides a method for making an aluminum alloy wherein during cold rolling interanneal is carried out at a gauge such that the cold work after internanneal is between about 30-70%.

BACKGROUND OF THE INVENTION

The present invention describes a method of forming an aluminum alloyfoil suitable for continuous casting in the fabrication of fins used inbrazed automotive heat exchangers.

Aluminum foil is the preferred choice for forming fins used in heattransfer and air conditioning equipment such as radiators, condensers,oil coolers and evaporators. A preferred choice of alloy for thisapplication, particularly in automobile heat exchangers, is the alloy AA3003. It has the composition shown in Table I below: TABLE I ElementsWt. % Silicon  0.6 max Iron  0.7 max Copper 0.05-0.20% Manganese 1.0-1.5 Zinc 0.10 maxBalance aluminum (including unavoidable impurities)

In another version of this alloy, Zn is also added to it when the fin isused in the sacrificial mode. The concentration of Zn can vary between0.5 to 2%.

The aluminum foil is usually in the thickness range of 0.002-0.008 in.An aluminum sheet is cold rolled to this final gauge starting from areroll sheet that is in the gauge range of 0.04-0.3 in. The reroll sheetis produced by casting DC ingots, homogenizing at elevated temperatures,and then hot rolling to the reroll gauge. Alternatively, the rerollsheet is produced by a continuous casting process and is directly hot orwarm rolled to the final reroll gauge. The continuous casting process isadvantageous because it is more productive and less expensive.

The radiators, evaporators and other heat exchangers are produced bybrazing aluminum fins to the clad aluminum sheet of different shapes orforms. The brazing operation takes place at around 600° C. At thistemperature aluminum fins can sag and collapse or, in some cases, affectthe crushing pressure that the finished units can take at ambient andoperating temperatures. The tendency to sag during brazing cycle ismeasured by a property that is characterized as sag resistance. It isgenerally accepted that higher sag resistance improves performance ofthe unit. Sag resistance in general is dependent upon the post brazegrain size of the aluminum foil. Higher grain size produces higher sagresistance. Therefore, higher grain size (>100 micron in transverse andlongitudinal directions) after annealing the sheet is preferred inautomotive fin applications.

Alloy 3003, particularly produced by continuous casting method, such asby a belt caster, yields significantly higher grain size than thatproduced from a corresponding hot rolled and homogenized DC material.The aluminum sheet can be used in the fin forming operation in differenttempers, such as from fully hard to fully annealed.

As stated before, alloy 3003 produced by continuous casting is anexcellent choice for auto-fin applications because it yields high postbraze strength and sag resistance. However, because it contains highamount of Mn and Cu, it gets easily work hardened and is thereforedifficult to roll. Further, during belt casting operation it yields ahighly convex profile (>0.6% crown) because it causes high temperaturedifferences between the center and edge of the sheet. This is due to thehigh Mn and Fe content of the alloy. The convex profile causes problemsduring rolling and the slitting operations that follow after rolling. Asa result, the cost of production of continuous cast 3003 is rather high.

Previous attempts have been made to develop aluminum alloy basedaluminum foils. For example, U.S. Pat. No. 4,906,534 (Bekki) discusses amethod of preparing thin aluminum plates used as fins of heat exchangersfrom an aluminum alloy core material, comprising 0.6 to 2.0 wt % of Mn,0.3 wt % or less of Fe, 0.05 to 0.6 wt % of Si, 0.5 to 2.0 wt % of Zn,0.05 to 0.2 wt % of Cu, and a balance of Al. The patent additionallycontemplates use of skin materials of an Al—Si system or an Al—Si—Mgsystem for cladding the surfaces of the core material.

U.S. Pat. No. 4,334,935 (Morris) discloses a fine grained, formableAl—Mn alloy sheet used to make rigid foil containers. The alloy consistsessentially of 1.3-2.3% Mn, up to 0.5% each of Fe, Mg, and Cu, up to0.3% Si, up to 2.0% Zn, less than 0.1% each of Zr, Cr, and Ti, otherelements up to 0.3% each and up to 1.0% total.

U.S. Pat. No. 5,888,320 (Dorward) describes a method of producingaircraft and automobile panels. The product includes an aluminum basealloy consisting essentially of about 0.6 to 1.4 wt. % silicon, not morethan about 0.5 wt. % iron, not more than about 0.6 wt. % copper, about0.6 to 1.4 wt. % magnesium, about 0.4 to 1.4 wt. % Zn, at least oneelement selected from the group consisting of about 0.2 to 0.8 wt. %manganese and about 0.05 to 0.3 wt. % chromium.

U.S. Pat. No. 5,725,695 (Ward) discloses an aluminum foil product madefrom an aluminum-silicon-iron aluminum alloy consisting essentially of0.30-1.1% Si, 0.40-1.0% Fe, max 0.10% Cu, max 0.10% Mn, max 0.05% Mg,max 0.05% Cr, max 0.10% Zn, and max 0.08% Ti.

U.S. Pat. No. 4,169,728 (Takeuchi) discloses an alloy for die-castingwhich consists essentially of 0.5-2.5% Zn, 1.1 to 3.0% Mg, 0.3 to 1.2%Si, 0.2 to 1.5% Fe, 0.3 to 1.2% Mn, and 0.1 to 0.3% Cu.

The disclosures of the foregoing references are incorporated byreference into this application in their entirety.

SUMMARY OF THE INVENTION

The disadvantages of prior methods and alloys may be overcome by thepresent invention, which provides an aluminum alloy foil for fins usedin heat exchangers. The improved alloy composition of the presentinvention consists essentially of about 0.25% to about 0.6% by weight ofSi, about 0.15% to about 0.50% by weight of Fe, about 0.20% to about0.70% by weight of Mn, less than about 0.05% Cu, and less than about0.05% Mg with the balance aluminum including unavoidable impurities. Thealloy composition may also contain less than 0.10% Zn or an amount of Znin the range from about 0.50 to 2.00% by weight.

The invention also provides a method for making an improved aluminumfoil. The method comprises providing a molten aluminum foil alloy havingthe composition stated in the previous paragraph. The molten alloy iscontinuously cast into an aluminum alloy strip from the molten aluminumalloy, and cold rolled into a final gauge of between about 0.002-0.008inches.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The present invention provides two new alloy compositions, one with Znand one without Zn. The improved alloy has lower density, reducedmanganese levels, and larger grain size to improve sag resistance. Thepreferred composition ranges for the aluminum alloy in accordance withthe present invention is shown in Table II, below: TABLE II Elements Wt.% Composition A Silicon 0.25-0.60 Iron 0.15-0.50 Manganese 0.20-0.70Copper <0.05 Magnesium <0.05 Zinc <0.10 Composition B Silicon 0.25-0.60Iron 0.15-0.50 Manganese 0.20-0.70 Copper <0.05 Magnesium <0.05 Zinc0.50 to 2.00Balance aluminum (including unavoidable impurities)

The alloy of the present invention has a lower density than DC cast3003. This reduces the cost of using the alloy as fin stock in brazedheat exchangers where surface area is the determining factor.

In a preferred embodiment of the invention, Si content ranges betweenabout 0.3 and about 0.5%. Silicon and intermetallic particles ofsilicon, iron and aluminum precipitate in the sheet, making the surfacehard. This reduces smut (reaction product) formation during coldrolling. Si above 0.6% is not desired because it makes the scrap lesscompatible with can scrap and therefore less desirable. Additionally,the amount of silicon present affects the smut generated during coldrolling. As it exceeds a critical level of 0.25%, smut generationdecreases significantly. This is because of precipitation of silicon andintermetallic particles of silicon within the matrix that increase thehardness of the metal and thereby reduce the smut generated during coldrolling.

In a preferred embodiment of the present invention, Fe content rangesbetween about 0.15 and about 0.35%. Low Fe helps increase grain size ofthe alloy after annealing because it reduces the amount of ironaluminide and iron silicon aluminide particles which pin grains andreduce grain size. Very low Fe is also undesirable as it can affect thetotal Fe+Mn content in the alloy. The Fe+Mn content needs to becontrolled within desired limits, for reasons explained below.

The reduced manganese levels of the alloy of the present inventionaccommodate a different strengthening mechanism compared to DC cast3003. In DC cast 3003, the Mn precipitates out during the hotrolling/homogenizing steps and the precipitates harden the metal. In thepresent alloy, the Mn stays in solution and provides a solutionstrengthening effect. For the composition ranges of the presentinvention, solution strengthening is more effective at producing astrong final product than precipitation hardening.

In a preferred embodiment of the invention, Mn content ranges betweenabout 0.30 and about 0.60%. Reducing Mn from the level present in 3003reduces the required amount of work hardening during cold rolling, andthereby helps improve cold rolling productivity. The lower limit for Mnis necessary because below this level the grain size after annealingdecreases to the level at which it can affect the sag resistance.

The optimal grain size for this application is larger than normal.Generally, small grains are preferred for a good combination offormability and strength. In this application, however, sag resistanceis more important and larger grain sizes improve sag resistance.

In a preferred embodiment of the present invention, Mn+Fe content is0.40%-0.80%. The lower limit is necessary because below this level theshrinkage during casting increases to such an extent that it affects theheat transfer during casting and therefore results in poor surface ofthe reroll. Poor reroll surface can result in surface cracks in thealloy. The center to edge temperature difference during casting iscorrelated to the amount of Mn+Fe in the alloy. When the amount of Mn+Feincreases, the temperature difference between center and edge alsoincreases. This results in poor profile during hot rolling, which inturn causes problems during cold rolling and slitting.

In a preferred embodiment of the present invention, Cu content is<0.05%. Copper content of the alloy is minimized because copper resultsin work hardening during cold rolling.

The alloys of the present invention are continuously cast to form anas-cast strip 30 mm (1.18 inches) or less in thickness. The alloys arepreferably cast using twin belt casting, block casting or twin rollcasting. In all cases the cast strip is not homogenized prior tosubsequent rolling. The as-cast strip may be optionally hot or warmrolled prior to cold rolling or may be directly cold rolled. Warmrolling is carried out at a temperature low enough to avoidrecrystallization.

Interanneal during cold rolling is carried out at a gauge such that thecold work after interanneal is between 30-70%. This results in largepost braze grain size. The lower limit is necessary as below this levelrolling speed in the final pass can decrease significantly.

The composition of the alloy is chosen such that it optimizesperformance during casting and rolling to obtain the best combination ofpost braze properties. Mn and Fe are two elements that affect castingperformance significantly. Aluminum shrinks during casting. Tocompensate for this shrinkage, the casting gap is reduced from thestarting to the finishing point. However, composition of individualalloys can also affect the amount of shrinkage. Two elements that canreduce the amount of shrinkage are Mn and Fe. Reduced shrinkage resultsin increased heat transfer during casting and therefore yields excellentsurface after rolling. Too low a shrinkage can result in increasedtransverse temperature difference which in turn leads to poor profileafter warm rolling. The two must be optimized to give a satisfactorysurface and profile.

Another aspect of this invention is the control of post braze grainsize. We have discovered that grain size increases with increasing Mnand decreases with increasing Fe. Accordingly, the two must be optimizedwhile respecting limits for improved casting performance. The inventionalso provides another mechanism for controlling grain size: the alloy iscold worked before brazing to increase the post braze grain size.

The present invention results in better rolling productivity, improvedprofile, and density of aluminum reduced by about one percent, whichyields more feet of sheet for the same weight and thereby improvesproductivity for the user. There could also be uses for the presentinvention in other sheet or foil applications.

It is to be understood that the invention is not limited to the featuresand embodiments set forth above but may be carried out in other wayswithout departure from its scope and spirit. Accordingly, it is intendedthat the present invention be limited only by the following claims.

1. An aluminum-based alloy consisting essentially of about 0.25% toabout 0.60% by weight of Si; about 0.15% to about 0.50% by weight of Fe;about 0.20% to about 0.70% by weight of Mn; less than about 0.05% Cu;and less than about 0.05% Mg, with the balance aluminum includingunavoidable impurities.
 2. The alloy of claim 1, wherein the alloycontains 0.10% by weight of Zn.
 3. The alloy of claim 1, wherein thealloy contains 0.50-2.00% by weight of Zn.
 4. The alloy of claim 1,wherein the alloy contains about 0.3-0.5% by weight of silicon.
 5. Thealloy of claim 1, wherein the alloy contains about 0.15-0.35% by weightof iron.
 6. The alloy of claim 1, wherein the alloy contains about0.30-0.60% by weight of manganese.
 7. The alloy of claim 1, wherein thealloy contains about 0.40-0.80% by weight of manganese and iron.
 8. Thealloy of claim 1 in the form of a cold rolled sheet, wherein during coldrolling interanneal is carried out at a gauge such that the cold workafter interanneal is between 30-70%.
 9. An aluminum foil made bycontinuously casting an aluminum alloy strip from a molten alloy havinga composition in accordance with claim 1, and cold rolling thecontinuous cast aluminum strip to a final gauge of between about0.002-0.008 inches.
 10. A heat exchanger having fins comprising an alloyhaving a composition in accordance with claim
 1. 11. A fin for a heatexchanger comprising an alloy having a composition in accordance withclaim
 1. 12-20. (Canceled)
 21. An aluminum foil made by continuouslycasting an aluminum alloy strip from a molten alloy having a compositionin accordance with claim 1, and cold rolling the continuous castaluminum strip to a final gauge of between about 0.002-0.008 inches,wherein the cold rolling step is carried out at a gauge such that thecold work after cold rolling is between 30% to 70%.
 22. A heat exchangerhaving fins comprising an alloy made by continuously casting an aluminumalloy strip from a molten alloy having a composition in accordance withclaim 1, and cold rolling the continuous cast aluminum strip to a finalgauge of between about 0.002-0.008 inches, wherein the cold rolling stepis carried out at a gauge such that the cold work after cold rolling isbetween 30% to 70%.
 23. A fin for a heat exchanger comprising an alloymade by continuously casting an aluminum alloy strip from a molten alloyhaving a composition in accordance with claim 1, and cold rolling thecontinuous cast aluminum strip to a final gauge of between about0.002-0.008 inches, wherein the cold rolling step is carried out at agauge such that the cold work after cold rolling is between 30% to 70%.